Central Northern Calcareous Alps Research Articles

Salt-rich versus salt-poor structural scenarios in the central Northern Calcareous Alps: implications for the Hallstatt facies and early Alpine tectonic evolution (Eastern Alps, Austria)

One of the most remarkable features of the central Northern Calcareous Alps (Eastern Alps, Austria) is the widespread presence of Upper Triassic deep-water carbonates (the Hallstatt facies) and Permo-Triassic evaporites resting on deep-water Middle Jurassic strata and their underlying Upper Triassic shallow-water carbonate platform successions. The Hallstatt facies and accompanying evaporites have been classically interpreted to originate either from a location south of the time-equivalent carbonate platforms, or to have been deposited in deeper water seaways within the broad platform domain. To date, this dispute has been addressed mostly through the analysis of Triassic and Jurassic facies distribution in map view, which, however, is subject to some degree of ambiguity and subjectivity. In this contribution we present, for the first time, sequentially restored regional cross-sections through the central Northern Calcareous Alps to understand the implications of the contrasting paleogeographic models. We present (a) an interpretation based on a highly allochthonous origin of the Triassic deep-water units and (b) an interpretation based on their relative autochthony in which we incorporate the potential influence of salt tectonics in the central NCA. The restored cross-sections provide a framework within which the alternative scenarios and their paleogeographic implications can be better understood. Through this analysis we propose that salt tectonics in the central NCA can provide a valid explanation for apparent inconsistencies in the relative autochthony scenario and thus constitutes a reasonable alternative to the currently accepted allochthony scenario.Graphical abstract

Deformation of the Dachstein Limestone in the Dachstein thrust sheet (Eastern Alps, Austria)

Abstract Deformation affecting the Upper Triassic Dachstein Limestone has been analyzed in the Dachstein thrust sheet, the uppermost thrust unit of the central Northern Calcareous Alps (Eastern Alps). Different scales of deformation are discussed, from kilometer-scale thrusting down to folds in the order of tens of meters to meters. Observations are based on both conventional outcrop observations and on digital fieldwork performed on drone-captured virtual outcrops and on GoogleMaps 3D terrain renderizations. The structures observed were formed at different times and document the following events: 1) Late Triassic syn-depositional instability and slumping; 2) Late Triassic syn-depositional growth of the Hallstatt diapir; 3) Late Triassic syn-depositional, salt-driven, extensional faulting; 4) Jurassic-age re-activation of extensional faults; 5) (presumably) Early Cretaceous shortening in both east-west and north-south directions; and 6) (presumably) Late Cretaceous extensional re-activation of faults. The structures and their origin have a bearing on the interpretation of the tectonic evolution of the Dachstein thrust sheet, highlighting the potential relevance of salt tectonics in controlling its structure.

Precursor extension to final Neo-Tethys break-up: flooding events and their significance for the correlation of shallow-water and deep-marine organisms (Anisian, Eastern Alps, Austria)

The Anisian depositional history in the Western Tethys realm provides the possibility to study the stepwise opening of the Neo-Tethys, and is well preserved in the sedimentary record of the Northern Calcareous Alps. Whereas the depositional characteristic in the Early (to early Middle) Anisian is characterized by shallow-water carbonates, formed in a semi-restricted environment, the situation changed in the Middle Anisian. A rapid increase of subsidence resulted in an abrupt deepening event with deposition of deeper-water limestones, in some cases even with chert nodules, sometimes with resedimented shallow-water debris intercalated in radiolaria-filament wackestones, or with clayey or marly intercalations. This abrupt deepening, termed the Annaberg Event, is followed by a shallow-water carbonate evolution. The deeper-water limestones can be dated by conodonts and shallow-water organisms like calcareous algae or foraminifera from the resedimented intercalations as Late Bithynian to Early Pelsonian. In contrast to the Early Anisian microbial carbonates, formed under semi-restricted conditions, the Middle Anisian (Pelsonian) shallow-water carbonates were formed under fully marine influence and a diverse fauna and flora was, therefore, able to counterbalance the rapid subsidence by increasing carbonate production. During Middle Anisian times, the newly tectonically created accommodation space became rapidly filled by shallow-water carbonates. At the end of the Middle Anisian (Late Pelsonian), the final break-up of the Neo-Tethys led to a rapid decrease of carbonate production and widespread deposition of deep-marine and condensed limestones. This drowning event (Reifling Event) was accompanied by the formation of a horst-and-graben morphology, dated by conodonts and ammonoids as late Middle Anisian from overlying condensed limestones. In contrast to the well-known drowning event in the late Middle Anisian, precursor events to the final oceanic break-up of the Neo-Tethys have not yet been described, but play an important role in the reconstruction of the opening history of the Neo-Tethys. This knowledge gap is filled by the analysis of exactly datable sedimentary successions in the central Northern Calcareous Alps.

A geological snapshot from the front of the Northern Calcareous Alps: Well Obermoos TH-1, Salzburg, Austria

The deep thermal well Obermoos TH-1 (total depth 2468 m, year 1990) was drilled within the Upper Cretaceous Salzburg-Reichenhall basin of the Northern Calcareous Alps at the southwestern edge of Salzburg city, Austria. The lithologic log shows c. 200 m thick Quaternary sediments of the glacially overdeepened Salzach Valley above bedrock. The Quaternary infill is underlain by c. 250 m thick, almost horizontally lying coarse clastics and marls, which belong to the Upper Cretaceous Salzburg-Reichenhall Gosau basin. From 456 m to 2468 m, a steeply dipping, more or less continuous succession from Cenomanian strata to Upper Triassic Hauptdolomit was encountered. This succession is considered being part of the Bajuvaric nappe of the central Northern Calcareous Alps, which is entirely overridden by units of the Tyrolic arc in the study area. The new data from the deep drilling provides (i) new information on depth and filling of the deeper parts of the Pleistocene valley and (ii) new insights into the complex structure of the Bajuvaric nappe. We also discuss a potential fault crossing the Salzburg-Reichenhall basin being part of the Cenozoic Innsbruck-Salzburg-Amstetten fault system. On a larger scale, the Tyrolic unit exposed in the southern margin of the Salzburg-Reichenhall basin and potentially overlying the borehole section must be the same as in the flat-laying Tyrolic nappe found in two deep drill holes, 15 km southeast and 18 km east of the City of Salzburg (Vigaun 1 and Vordersee 1).

Ophiolitic detritus in Kimmeridgian resedimented limestones and its provenance from an eroded obducted ophiolitic nappe stack south of the Northern Calcareous Alps (Austria)

Abstract The causes for the Middle to Late Jurassic tectonic processes in the Northern Calcareous Alps are still controversially discussed. There are several contrasting models for these processes, formerly designated “Jurassic gravitational tectonics”. Whereas in the Dinarides or the Western Carpathians Jurassic ophiolite obduction and a Jurassic mountain building process with nappe thrusting is widely accepted, equivalent processes are still questioned for the Eastern Alps. For the Northern Calcareous Alps, an Early Cretaceous nappe thrusting process is widely favoured instead of a Jurassic one, obviously all other Jurassic features are nearly identical in the Northern Calcareous Alps, the Western Carpathians and the Dinarides. In contrast, the Jurassic basin evolutionary processes, as best documented in the Northern Calcareous Alps, were in recent times adopted to explain the Jurassic tectonic processes in the Carpathians and Dinarides. Whereas in the Western Carpathians Neotethys oceanic material is incorporated in the mélanges and in the Dinarides huge ophiolite nappes are preserved above the Jurassic basin fills and mélanges, Jurassic ophiolites or ophiolitic remains are not clearly documented in the Northern Calcareous Alps. Here we present chrome spinel analyses of ophiolitic detritic material from Kimmeridgian allodapic limestones in the central Northern Calcareous Alps. The Kimmeridgian age is proven by the occurrence of the benthic foraminifera Protopeneroplis striata and Labyrinthina mirabilis, the dasycladalean algae Salpingoporella pygmea, and the alga incertae sedis Pseudolithocodium carpathicum. From the geochemical composition the analysed spinels are pleonastes and show a dominance of Al-chromites (Fe3+–Cr3+–Al3+ diagram). In the Mg/(Mg+ Fe2+) vs. Cr/(Cr+ Al) diagram they can be classified as type II ophiolites and in the TiO2 vs. Al2O3 diagram they plot into the SSZ peridotite field. All together this points to a harzburgite provenance of the analysed spinels as known from the Jurassic suprasubduction ophiolites well preserved in the Dinarides/Albanides. These data clearly indicate Late Jurassic erosion of obducted ophiolites before their final sealing by the Late Jurassic–earliest Cretaceous carbonate platform pattern.

The Triassic–Jurassic boundary in the shallow-water marine carbonates from the western Northern Calcareous Alps (Austria)

The Triassic–Jurassic (T–J) boundary coincides with one of the five biggest mass extinctions in the Phanerozoic. This event has been extensively studied in the eastern and central Northern Calcareous Alps (NCA), where the global stratotype section and point for the base of the Jurassic is outcropping. We present one of the first combined bio-, litho- and carbonate carbon isotope stratigraphic study across the T–J boundary from the western NCA, focusing on the shallow-water marine carbonate successions “Loruns” and “Steinernes Meer”. An almost complete T–J succession with the Upper Triassic Kossen Formation, the T–J Schattwald beds and the Lower Jurassic Loruns oolite is preserved in the Loruns section, whereas a stratigraphic hiatus at the T–J boundary is observed in the Steinernes Meer section. The carbonate carbon isotope record of Loruns shows the characteristic T–J carbon isotope evolution with a short initial negative carbon isotope excursion (CIE) in the lower Schattwald beds, which is separated from the following and longer main negative CIE in the Loruns oolite by a positive excursion in the upper Schattwald beds. The synchroneity of the initial and main CIE with the last occurrence of Triassic fauna and the first appearance of Jurassic ammonites suggests placing the T–J boundary at the base of the Loruns oolite. The Steinernes Meer section records only the main negative CIE in the Loruns oolite due to emersion and/or no sedimentation between the Upper Triassic Kossen Formation and the Lower Jurassic Loruns oolite, caused by regression and synsedimentary tectonics. This study provides new evidence in support the hypothesis that the latest Rhaetian decrease and the Hettangian recovery of the carbonate production coinciding with the initial and main CIE, respectively, can be interpreted as evidence of acidification of the Tethys ocean due to elevated atmospheric CO2 concentrations, probably caused by voluminous volcanic gas emissions.

The Haselgebirge evaporitic mélange in central Northern Calcareous Alps (Austria): Part of the Permian to Lower Triassic rift of the Meliata ocean?

For the reconstruction of Alpine tectonics of the Eastern Alps, the evaporitic Permian to Lower Triassic Haselgebirge Formation plays a key role in (1) the origin of Haselgebirge bearing nappes, (2) the inclusion of magmatic and metamorphic rocks revealing tectonic processes not preserved in other units, and (3) the debated mode of emplacement of the nappes, namely gravity-driven or tectonic. Within the Moosegg quarry of the central Northern Calcareous Alps gypsum/anhydrite bodies are tectonically mixed with lenses of sedimentary rocks and decimeter- to meter-sized tectonic clasts of plutonic and subvolcanic rocks and rare metamorphics. We examined various types of (1) widespread biotite–diorite, meta-syenite, (2) meta-dolerite and rare ultramafic rocks (serpentinite, pyroxenite) as well as (3) rare metamorphic banded meta-psammitic schists and meta-doleritic blueschists. The apparent 40Ar/39Ar biotite ages from three biotite–diorite, meta-dolerite and meta-doleritic blueschist samples with variable composition and fabrics range from 248 to 270Ma (e.g., 251.2±1.1Ma) indicating a Permian age of cooling after magma crystallisation or metamorphism. The chemical composition of biotite–diorite and meta-syenite indicates an alkaline trend interpreted to represent a rift-related magmatic suite. These, as well as Permian to Jurassic sedimentary rocks, were incorporated during Cretaceous nappe emplacement forming the sulphatic Haselgebirge mélange. The scattered 40Ar/39Ar white mica ages of a meta-doleritic blueschist (of N-MORB origin) and banded meta-psammitic schist are ca. 349 and 378Ma, respectively, proving the Variscan age of pressure-dominated metamorphism. These ages are similar to detrital white mica ages reported from the underlying Rossfeld Formations, indicating a close source–sink relationship. According to our new data, the Haselgebirge bearing nappe was transported over the Lower Cretaceous Rossfeld Formations, which include many clasts derived from the Haselgebirge Formation and its exotic blocks deposited in front of the incoming nappe comprising the Haselgebirge Formation.

Spatial and temporal development of siliceous basin and shallow-water carbonate sedimentation in Oxfordian Northern Calcareous Alps

The Late Jurassic succession of Mount Rettenstein (central Northern Calcareous Alps, Austria) is unique in comparison to all other sections known in the Northern Calcareous Alps because it provides the oldest coexistence of radiolarite basin sedimentation with contemporaneous shallow-water carbonate intercalations. An up to 3.5-m-thick debris flow made up of shallow-water carbonate detritus with a radiolaritic matrix is overlain by thin (calcareous) radiolarite, followed by several hundreds of meters of shallow-water carbonates of the Plassen Formation. Benthic foraminifers (Labyrinthia mirabilis Weynschenk and Alveosepta aff. jaccardi) and the radiolarian associations indicate a depositional age of both the debris flow and the basal Plassen Formation around the boundary of Middle/Late Oxfordian resp. in the Late Oxfordian. This is as yet the first unambiguous evidence of Oxfordian shallow-water sedimentation in the Northern Calcareous Alps. This early neritic stage with the settlement of ooid bars and coral-stromatoporoid-reefs, evidenced by the debris flow resediments in siliceous basin sedimentation, is followed by the definite, rapid progradation of the actual Late Oxfordian/Kimmeridgian–Berriasian Plassen Carbonate Platform with its steep slope configuration. Assumably, this evolution was steered by a mixture of both global environmental and regional tectonic constraints.

New evidence for block and thrust sheet rotations in the central northern Calcareous Alps deduced from two pervasive remagnetization events

We present 81 paleomagnetic sites (Early Triassic to Early Cretaceous) from the central sector of the northern Calcareous Alps (NCA, Eastern Alps, Austria and Germany). Stepwise thermal demagnetization defines three magnetic directions mostly carried by low unblocking temperature and low‐coercivity minerals: J3, 350°; J2, 500°; J1, 575° and 680°. J3 and J2 show positive inclinations, whereas J1 (very seldom) is of dual polarity. The fold tests show that a J3 can be interpreted as a postfolding and posttilting remagnetization and J2 as a postfolding and pre(syn)tilting. J1 can be considered as primary because of the occurrence of two polarities and evidence presented by other authors in the area. All three components show a systematic and significant clockwise rotation after comparing with the expected European references. J2 or J1 are marked by higher rotation values than J3. J2 shows different inclinations depending on the structural position (north or southward dips). Considering the structural evolution and the observed inclinations, the first postfolding and pretilting remagnetization event (J2) could have taken place between Late Cretaceous and Eocene times but certainly before the thrusting of the NCA over the Rhenodanubian Flysch and northward tilting caused by the stacking of the lower Austroalpine nappes. The second postfolding and posttilting remagnetization (J3) would have been acquired after the final thrusting of the NCA over Penninic units. From then, the NCA behaved as a set of rigid blocks recording the main stage of vertical axis clockwise rotation (65° in average) associated with the continental collision. The variable degree of rotation in the different positions (from 40° to 134°) can be explained by individual vertical axis rotation in a block system trying to adjust to space problems. The constant declination differences between J2 and J3 (25° in average) would reflect the rotation related with the lateral differences of shortening during the oblique thrust of the Austroalpine units over the Penninic units.

The Late Jurassic to Early Cretaceous sedimentary succession of the Hoherstein-Sandling area (Salzkammergut, Austria) implications for the reconstruction of the block puzzle in the central Northern Calcareous Alps

The Late Jurassic to Early Cretaceous sedimentary succession of the Hoherstein-Sandling area (Salzkammergut, Austria) implications for the reconstruction of the block puzzle in the central Northern Calcareous Alps

The nappe structure of the central Northern Calcareous Alps and its disintegration during Miocene tectonic extrusion?a contribution to understanding the orogenic evolution of the Eastern Alps

The classical concept of the nappe structure of the central Northern Calcareous Alps (NCA) is in contradiction with modern stratigraphic, structural, metamorphic and geochronological data. We first perform a palinspastic restoration for the time before Miocene lateral tectonic extrusion, which shows good continuity of structures, facies and diagenetic/metamorphic zones. We present a new nappe concept, in which the Tirolic unit practically takes the whole area of the central NCA and is divided into three subunits (nappes): Lower and Upper Tirolic subunit, separated by the Upper Jurassic Trattberg Thrust, and the metamorphic Ultra-Tirolic unit. The Hallstatt (Iuvavic) nappe(s) formed the highest unit, but were completely destroyed by erosion after nappe stacking. Remnants of the Hallstatt nappes are only represented by components of up to 1 km in size in Middle/Upper Jurassic radiolaritic wildflysch sediments ("Hallstatt Melange" belonging to the Tirolic unit). Destruction of the continental margin started in Middle to Upper Jurassic time and prograded from the oceanic side towards the shelf. The original substratum of the external nappes (Bavaric units) of the NCA was largely the Austroalpine crystalline basement, of the internal nappes (Tirolic units) the weakly metamorphosed Palaeozoic sequences (Greywacke Zone and equivalents). Eocene movements caused limited internal deformation in the Tirolic unit.

Paleomagnetism and paleogeography of the northern calcareous Alps (Austria)

Prefolding magnetization components have been resolved at 38 sites of Liassic “Adnet” limestones in the central Northern Calcareous Alps (NCA) and at one site in the Drauzug. The site mean directions are northeasterly and down, and are closer to coeval European directions than to Southern Alpine “African” directions. The western end of both the NCA and of the Drauzug yield different site mean directions and negative fold tests indicating remagnetization. The sampling localities have been rotated relative to one another during thrusting, however, comparison of paleomagnetic poles with the European apparent polar wander path implies an average clockwise rotation of about 30° for the NCA relative to Europe. Larger‐scale clockwise rotations in excess of 80°, relative to the Southern Alps, are implied by comparison with the Southern Alpine (African) polar wander path. Facies analysis of Jurassic and Lower Cretaceous basinal sediments in the central part of the NCA suggests that Liassic subsidence was related to presently E‐W sinistral strike‐slip faults, whereas Malm‐Berriasian subsidence was controlled by presently E‐W south‐dipping normal faults. Both paleomagnetic and facies data tend to indicate that the NCA did not undergo the “African” rotation seen in Southern Alpine paleomagnetic data. This implies the existence of a short‐lived tract of Triassic?‐Jurassic ocean floor between the NCA and the Southern Alps, which linked the Pindos Ocean to the Ligurian trough and allowed Adria to rotate independently from the NCA during the Late Jurassic. Eo‐Alpine deformation and metamorphism are associated with the Early Cretaceous closure of this “Hallstatt” trough.

Faziesentwicklung an der Trias/Jura-Wende in den mittleren Nördlichen Kalkalpen

Faziesentwicklung an der Trias/Jura-Wende in den mittleren Nördlichen Kalkalpen